Electrolysis of alkyl grignardcontaining electrolytes



April 1964 J. M. COOPERSMITH 3,131,135

ELECTROLYSIS OF ALKYL. GRIGNARD-CONTAINING ELECTROLYTES Filed Jan. 23,1961 Fig. 2

1N VEN TOR.

John M. 000 ersmifh az d ATTORNEY Patented Apr. 28, 1964 3,131,135ELECTROLYSES OF ALKYL GRIGNARD- CONTAlNlNG ELECTROLYTES John M.Coopersmith, Olympia Fields, 111., assignor to Standard Oil Company,Chicago, 111., a corporation of Indiana Filed Jan. 23, 1961, Ser. No.84,209 4 Claims. (Cl. 204-59) This invention relates to electrolyticcells used in chemical conversion processes, and more particularlyconcerns a novel cooling system for such cells when employed in theelectrolysis of alkyl Grignard-containing electrolytes.

It is heretofore known that organometallic compounds of such metals aszinc, aluminum, magnesium, cadmium, tin, lead, and others may beprepared by electrolyzing a Grignard reagent with an anode composed ofthe metal which is to form the organometallic compound. Such processes,by reason of their versatility, simplicity, and economy, are nowbecoming of increasing value. Nevertheless, certain construction andoperating problems remain, and it is an object of the present inventionto provide a unique solution to what has heretofore been the mosttroublesome of these.

When an electrolysis is being conducted, it is often desirable tomaintain electrolyte temperature within a fairly close range. Thisbecomes quite diflicult in the electrolysis of alkyl Grignardelectrolytes due to the comparatively high 1 R losses, coupled withunavoidable heat of reaction. It has heretofore been proposed to coolelectrolytic cells by employing various coolants in coils or tubesdisposed within the cell; however, the most convenient coolant, water,reacts violently with Grignard reagents to evolve heat and large amountsof hydrocarbon gas. Unfortunately, leakage of cooling water into theelectrolyte is altogether too likely to occur in view of the thincooling tubes and the corrosiveness of both water and the Grignardelectrolyte. It is, therefore, a primary ob iect of the presentinvention to provide a cooling system for such cells wherein any leakagebetween coolant and electrolyte will be substantially completelyinnocuous, and which, at all events, will not be hazardous tosurrounding personnel.

Briefly, in accordance with the invention, an electrolytic cellemploying an alkyl Grignard electrolyte and a consumable metal anode iscooled by indirect heat exchange with a coolant stream composed of thealkyl halide which corresponds to the alkyl Grignard electrolyte. Thatis, for example, if ethyl magnesium chloride is electrolyzed with a leadanode to produce tetraethyl lead, the electrolyte is cooled withcirculating ethyl chloride. Thus, should there by any leakage, thecoolant and electrolyte will be non-reactive.

In one aspect of the invention, the alkyl halide is maintained in theliquid phase, and is itself cooled by external heat exchange with anextraneous coolant such as water, which is not, however, permitted tocome in the vicinity of the electrolyte. This embodiment has theadvantage of economy of installation and simplicity of operation.

In another aspect, liquid alkyl halide is supplied to cooling coilsdisposed within the electrolyte, and is permitted to vaporize undercontrolled pressure. Thus heat produced during electrolysis is absorbedas latent heat of evaporation. The resultant alkyl halide vapors arethen recompressed, cooled and condensed, and the resultant liquid isthen available for return to the coils. It will be recognized that thisembodiment is, in effect, a vapor compression type refrigeration system,and offers the advantages of large heat capacity for a relatively smallcoil size.

The invention will be more fully described and clearly understood in theensuing description, which is to be read in conjunction with theattached drawings wherein:

FIGURE 1 schematically depicts an electrolytic cell cooled byrecirculating liquid alkyl halide; and

FIGURE 2 schematically shows an elevation of a cell cooled by the vaporcompression refrigeration system utilizing alkyl halide as refrigerant.

Before proceeding with a discussion of the respective figures, a fewwords may be said about the electrolytic process in general.Essentially, alkyl groups of from 1 to, say, 8 or more carbon atoms inan alkyl Grignard reagent, e.g. methyl magnesium chloride or ethylmagnesium chloride, are transferred by electrolysis to the con sumablemetal of the anode. As a result, the consumable anode metal, for examplelead, is converted to the corresponding metal alkyl, e.g. tetramethyl ortetraethyl lead.

The alkyl Grignard electrolyte comprises the alkyl Grignard reagenttogether with an appropriate solvent. Known Grignard solvents requirethe presence of an ether or tertiary amine, such as diethyl ether,dimethyl ether of ethylene glycol, dibutyl ether of diethylene glycol,hexylethyl ether of diethylene glycol, triethylamine, etc. An additionalether, for example tetrahydrofuran, is often found to be of advantagefor increasing electrolyte conductivity, as is the presence of anormally liquid aromatic hydrocarbon such as benzene, toluene, xylenes,etc. Lastly, the electrolyte may contain excess alkyl halide to reactwith magnesium metal plating out at the anode and thereby reconvert itto additional Grignard reagent. By way of example, a typical alkylGrignard electrolye prior to electrolysis may have an alkyl Grignardreagent concentration of about 1.5-3.5 Normal, an ether concentration ofabout 3080 weight percent, about 1040% tetrahydrofuran, about 20-50weight percent benzene, and about 1-10 weight percent excess alkylhalide.

Conditions within a typical electrolytic cell depend largely upon celldesign and considerations of thermodynamics and economics; consequently,they will vary widely depending upon the particular system. For thepreparation of tetraethyl lead, a temperature within the range of about20 C. to about (1., preferably about 2050 C. and optimally about 25-35C. is desired, while a current density at both anode and cathode withinthe range of about 0.2 to about 100 amperes per square foot of electrodearea is disadvantageous. Relatively low voltages, of the order of about20-30 volts, are usually preferred, although cell voltages of 50 voltsand even higher may be utilized. Cell pressures may range fromsubatmospheric to low superatmospheric, say 60 p.s.i.g., and theelectrolysis may be conducted batchwise, continuously, or semi-batchwisewith intermittent or continuous addition of reactants. It isparticularly noted that the inventive cooling system is suitable for anyand all of the foregoing variations, which illustrates its exceptionalversatility.

Turning now to FIGURE 1, an embodiment of the invention is depicted insimplified cross-sectional elevation. The cell proper comprises an outerjacket 11 containing electrolyte 12, which is admitted via conduit 13and withdrawn via conduit 14. Anode 16, of lead or other consumablemetal, is connected in circuit with cathode 17, of steel or otherconductive material, and with a direct current power source 18. It willbe appreciated that numerous arrangements of electrodes may be employed,including, without limitations, a plurality of flat plates, foraminousbaskets containing lead shot, etc.

A coil or tube 19 is disposed in electrolyte 12 in indirect heatexchange contact; that is, coolant within coil 19 can conduct heat fromelectrolyte 12 and thereby reduce or maintain the temperature duringelectrolysis.

Coil 19 may be finned, wound into a plurality of return bends, connectedwith similar coils or tubes in parallel or series-parallel arrangements,or otherwise constructed to afford a relatively high surface area.

Coolant in coil 19 is passed via line 21 and pump 22 through heatexchanger 23, where the alkyl halide coolant is itself cooled byindirect heat exchange with an external coolant, as for example water,and then passes via line 24 to coil 19.

A suflicient pressure is maintained on the alkyl halide coolant in coil19 to maintain substantially all of the alkyl halide in the liquidphase. However, should the cell be run above capacity, it may bedesirable to dispose a pre-cooler or condenser in line 21 upstream ofpump 22 in order to avoid presenting any vapor to pump 22 and therebyavoid pumping difliculties.

It will be noted that any leakage in coil 19 would merely result inalkyl halide coolant flowing into electrolyte 12, or vice versa,depending upon the relative pressures in coil 19 and in the electrolysiszone. Should pressure in coil 19 exceed that of the electrolysis zone,alkyl halide would leak into electrolyte 12, and would merely increasethe volume and pressure thereof, while not causing any serious chemicalor thermal disruptions. Similarly, should electrolyte leak into coil 19,there will be no adverse reaction produced.

Turning now to FIGURE 2, the second or vapor compression refrigerationtype embodiment is depicted. Identical elements in FIGURES 1 and 2 aredesignated with identical numbers. However, in this embodimentsubstantially all of the alkyl halide admitted to coil 29 is permittedto evaporate, and the resultant alkyl halide vapor is withdrawn viavapor line 31.

Tracing the flow of alkyl halide coolant in FIGURE 2, liquid alkylhalide, at a temperature equal to or below the boiling point at theparticular pressure prevailing in coil or tube 29, is transported tocoil 29 which is in heat exchange relationship with electrolyte 12. Asthe latter becomes heated during electrolysis, alkyl halide boils 01f.The resultant vapors pass through line 31 to back pressure control valve32, which maintains a constant or controlled back pressure on line 31and tube 29. Excess alkyl halide, over and above that necessary tomaintain the constant back pressure, is conducted as a vapor via line 33to compressor 34, which increases its pressure to that prevailing intube 29. Compressed vapor leaving compressor 34 passes through line 36and into heat exchanger 37, which functions as a condenser to condensethe compressed vapor and provide a liquid recycle stream which collectsin receiver 38 and is then sent to coil 29 via line 39. 7

Since the alkyl halide coolant or refrigerant in this FIGURE 2embodiment is selected to correspond with the alkyl Grignard electrolyte12 (e.g. methyl chloride for the electrolysis of methyl magnesiumchloride), the pressure maintained by back pressure control valve 32 isselected to provide a desired constant temperature in coil 29, and thusa substantially constant temperature in electrolyte 12. Relationshipsbetween pressure and boiling point of the several alkyl halides may befound in the literature, and are given, for example, in Perrys ChemicalEngineers Handbook, second edition, section 23 (McGraW-Hill, 1941);Physical Properties of Chemical Compounds, II, page 191 et seq.,Advances in Chemistry Series (American Chemical Society, 1959); andTimmermans Physico-Chemical Constants of Pure Organic Compounds, page210 et seq. (Elsevier, 1950). Thus, for example, with methyl chloride, atemperature of 25.0 C. is maintained in coil 29 by setting back pressurecontrol valve 32 to maintain a pressure of 1199 millimeters mercuryabsolute on line 31.

The embodiment of FIGURE 2 is particularly advantageous in that itpermits a relatively small tube or coil 29 to remove large quantities ofheat from the electrolysis zone due to the relatively high heat ofvaporization of alkyl halides. In addition, the vapor pressures ofboiling alkyl halides at temperatures most highly desired for conductingelectrolyses of alkyl Grignard electrolytes are generally fairly closeto those prevailing in the electrolytic cell, and consequently evenshould there be any leakage in coil 12 only a small amount of alkylhalide or electrolyte will leak through.

Thus it is evident that an extremely useful improvement in electrolyticcell design has been provided according to the invention. By selectingas a coolant the alkyl halide corresponding to the alkyl Grignardelectrolyte, any leakage between the two is rendered innocuous, andpermits electrolysis to continue with no hazard to personnel in the areaor to equipment. Furthermore, should a leak be detected, it isfrequently possible to provide sufiicient alkyl halide to the coolingcoil so that the alkyl halide coolant will leak into the electrolyte inan amount corresponding to the normal utilization of alkyl halide in theelectrolyte.

While the invention has been described in conjunction with specificembodiments thereof, it will be understood that these are exemplaryonly, and that many alternatives, modifications, and variations will beapparent to those skilled in the art in light of the foregoingdescription. For example, other alkyl halides may be used withcorresponding other Grignard reagents, e.g. methyl bromide ,ethyliodide, n-propyl chloride, n-propyl bromide, n-butyl chloride, etc.Accordingly, it is intended to embrace all such alternatives,modifications, and variations as fall within the spirit and broad scopeof the appended claims.

I claim:

1. In an electrolytic process employing an alkyl Grignard electrolytewherein heat is produced during electrolysis, the method of removingheat from the electrolysis zone which comprises passing a coolant streamof alkyl halide corresponding to the alkyl Grignard electrolyte inindirect heat exchange relationship with said electrolyte.

' 2. In an electrolytic process employing an alkyl Grignard electrolytewherein heat is produced during electrolysis, the method of removingheat from the electrolysis zone which comprises passing a coolant streamof alkyl halide corresponding to the alkyl Grignard electrolyte inindirect heat exechange relationship with said electrolyte, withdrawingsaid stream, cooling said stream by indirect heat exchange with anexternal coolant, and recycling said stream.

3. In an electrolytic process employing an alkyl Grignard electrolytewherein heat is produced during electrolysis, the method of removingheat from the electrolysis zone which comprises passing a liquid coolantstream of alkyl halide corresponding to the alkyl Grignard electrolytein indirect heat exchange relationship with said electrolyte, permittingsaid stream to vaporize under controlled pressure, withdrawing theresultant alkyl halide vapors, compressing and cooling said vapors tocondense the same, and recycling the resultant liquid.

4. In an electrolytic process for the preparation of tetraalkyl leadcompound, wherein an alkyl Grignard electrolyte is electrolyzed with alead anode, the method of removing heat from the electrolysis zone whichcomprises passing a coolant stream of alkyl halide corresponding to thealkyl Grignard electrolyte in indirect heat exchange relationship Withsaid electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS UNITEDSTATES PATENT oEEEEE @ERTIFICATE 0F RECTION" Patent No., 3, 131, 135Apri 1 28 1964 John M. Coppersmith It is hereby certified that errorappears in the above numbered patent req'liring correction and that thesaid Letters Patent should read as corrected below.

Column .2 line 47, for "disadvantageous read advantageous Signed andsealed this 18th day of August 1964.

(SEAL) Attest:

ERNEST wo SWIDER EDWARD -J. BRENNER Atmsting Officer Commissioner ofPatents

1. IN AN ELECTROLYTIC PROCESS EMPLOYING AN ALKYL GRIGNARD ELECTROLYTEWHEREIN HEAT IS PRODUCED DURING ELECTROLYSIS, THE METHOD OF REMOVINGHEAT FROM THE ELECTROLYSIS ZONE WHICH COMPRISES PASSING A COOLANT STREAMOF ALKYL HALIDE CORRESPONDING TO THE ALKYL GRIGNARD ELECTROLYTE ININDIRECT HEAT EXCHANGE RELATIONSHIP WITHSAID ELECTROLYTE.